The present invention relates to wind turbines, and more particularly to teeter control systems for wind turbines.
Wind turbines for converting wind energy to electrical energy typically comprise a rotor with one or more blades and a hub. The rotor is attached to, and supported by, a main shaft receiving the rotational power from the rotor and transmitting this power to a generator. The most popular type of large-scale (e.g., multi-megawatt) wind turbines orient the main shaft in a horizontal direction, thereby making the rotational plane of the rotor lie in essentially a vertical direction.
Most contemporary horizontal axis wind turbines use a three-bladed rotor, and fixedly attach the rotor to the main shaft. Accordingly, bending loads on the blades (i.e., loads in a direction substantially perpendicular to a plane of rotation of the rotor) are transmitted to the main shaft. These bending loads originate from uneven wind distribution over the swept area of the rotor, and due to gyroscopic forces associated with the mass of the rotor when the rotor and a nacelle are yawed away from the wind direction. The shaft and supporting structure is, thereby, built according to the weight and strength required to support these loads.
Since the early 1930's, some large-scale wind turbines have employed rotors with one or two blades, with the distinction that the rotor is attached to the shaft through a pin, called a teeter pin, which allows the rotor to move perpendicular to a time-averaged rotational plane of the rotor, thereby eliminating the transmission of bending loads to the main shaft (when the teetering motion is unconstrained).
An angle between the rotor blades at a given moment and the time-averaged plane of rotation (essentially a vertical plane) is called the teeter angle (β). During normal operation, teeter-angle variation is desirable: the teeter angle β varies within a certain range which can be denoted as a standard operating range, and, within that range, changes in response to wind-shear (which produces unequal wind velocity over the rotor swept area) and turbulence, and in response to gyroscopic forces produced by yawing the rotor into and away from a current wind direction. Due to a lag between load and displacement, maximum teeter-angle values for a two-bladed rotor typically occur when a rotor azimuthal position is essentially horizontal (i.e., parallel to the ground). At and around this horizontal rotor azimuthal position, there is no chance of collision between a blade and the tower (i.e., a blade-tower strike). Only when the rotor is in a vertical azimuthal position, does a blade pass in the vicinity of the tower. Consequently, the acceptable range of teeter-angle excursions depends on the azimuthal position of the rotor.
Teetering motion of the rotor reduces bending forces on the rotor that would otherwise be present and would cause fatigue in the blades, hub, and main shaft. There are two limits imposed on the teeter angle. The first limit is imposed by the mechanical structures at the rotor to main-shaft junction. The other, more constraining limit is due to blade-tower collisions. That is, if the teeter angle β increases past a certain value as a blade is passing near the wind-turbine support tower, there is risk of catastrophic blade-tower collision. To avoid this type of event, most turbines with teetering rotors include a teeter-restraint mechanism that prevents unwanted excursions of the teeter angle.
Two types of teeter restraint mechanisms are found in the prior art. One, which is called the contact type, consists of some flexible material, such as an elastomer or a metal spring, that becomes compressed once the rotor teeter angle exceeds a predetermined amount and contact between the rotor and the teeter restraint mechanism occurs. The restoring force imparted by this type of contact restraint mechanism onto the rotor is quite large, and “impulsive” in nature. These restraining loads are undesirable because they promote fatigue and catastrophic damage, thereby necessitating increased strength and weight in the rotor and nacelle structure. Furthermore, this type of restraint mechanism is independent from the rotor azimuthal position, therefore it provides unnecessary and damaging restraining force irrespective of rotor azimuthal position, and hence generates restraining forces even in the absence of any risk of blade-tower strike.
The other type of known teeter restraint mechanism uses a hydraulic cylinder, regulated by a control system, to provide a non-impulsive force restraining teeter motion. With this type of mechanism, teetering motion moves the piston within the cylinder, thereby displacing hydraulic fluid into a circuit external to the cylinder. The circuit connects at least two cylinders, so that the fluid ejected by one cylinder is accepted into the other. Restriction of teeter motion is generated by making the hydraulic fluid pass through a constriction, or orifice, located in this circuit. Because the pressure loss across the orifice increases with flow rate, this mechanism provides a teeter-restraint force that is proportional to, and only to, the teeter angular velocity, rather than to the teeter angle itself. This behavior is undesired, because most often, maximal angular velocity occurs as the rotor crosses a teeter angle β of zero degrees. Therefore, this second type of teeter restraint mechanism places a large, often maximal, restraining force on the rotor when the rotor is at zero teeter angle β, well within the standard operating range, and precisely when the possibility of tower strike is minimal. This restraining force is cyclic (occurring at every rotor revolution) and produces an unnecessary and damaging (e.g., fatigue-inducing) load on the rotor and the main shaft. Furthermore, large and beneficial teeter angular velocities also occur during nacelle-yaw maneuvers, wherein the unconstrained teeter-angle variation prevents large gyroscopic forces from reaching the main shaft. The second type of mechanism device resists, and fights against these rapid and beneficial teetering motions. In summary, the second type of mechanism imposes a restraining force on the rotor in conditions when free teeter motion is desired, including teeter angles inside the standard operating range, and teeter-angle excursions during yaw maneuvers, thereby reducing, if not eliminating, the fundamental benefits of the teetering rotor design.
In addition, known teeter restraint mechanisms lack means to prevent any teetering motion at desired times. For example, prior art teeter restraint mechanisms do not allow teetering motion to be blocked during start-up and during parked conditions when the rotor is not rotating.